1. Field of the Invention
The present invention relates to a scanning angle expander for expanding the angular extent of a scanning system of the type used in optical inspection equipment and more particularly to an scanning angle expander of the type used to inspect semiconductor wafers and the like. The invention is particularly useful for optically scanning patterned semiconductor wafers used in producing integrated-circuit dies or chips, and the invention is therefore described below particularly with respect to this application. The present invention more particularly relates to a scanning angle expanding device that increases the scanning angle of an optical scanner while maintaining a same beam diameter for the input and output optical beams of the scanning device.
2. Description of the Related Art
The inspection of semiconductor wafers is typically performed by scanning a laser beam across a wafer""s surface and collecting light scattered therefrom. The scanning operation is conducted by scanning the laser beam across the wafer surface in a first direction using one of a variety of known deflectors, such as acousto-optic deflectors or electromechanical deflectors, while moving a stage that supports the wafer thereon in a second direction, that is typically orthogonal to the first direction.
There are various devices and methods for scanning a laser beam, such as acousto-optic devices, electromechanical deflectors, and the like. There is a greater emphasis on the throughput of inspection device and accordingly on the throughput of scanners, as the design rules for semiconductors rapidly shrink without a corresponding decrease of the inspection sequence time period or the overall size of semiconductor dies or wafers.
A scanner is usually followed by a focusing unit that ideally focuses the scanned beam light to a spot onto the surface of the wafer. The focusing unit is characterized by its focusing abilities (which is commonly measured by its Numeric Aperturexe2x80x94NA) and the scanner is characterized by various parameters, such as its scan period and the angular extent of the scan. The resolution of the overall system (including the scanner and the focusing unit) is determined by the NA of the focusing unit and the size (or more particularlyxe2x80x94the cross section) of the light beam that exits the scanner and enters the focusing unit. The resolution (also termed xe2x80x9cspot sizexe2x80x9dxe2x80x94reflecting the size of light beam on the surface of the waferxe2x80x94after being focused) is inversely proportional to the size of the scanned beam prior to being focused by the focusing lens.
It is known in the art that a mere increment of throughput may deteriorate the system""s resolution. For example, the throughput may be incremented by increasing the angular extent of the scan while maintaining the same scan period. This increment may be achieved by passing a scanned beam through a telescope that includes an input lens having a first focal length F1 and an output lens having a second focal length F2 (whereas F2 is greater than F1). If the angular extent of the scan of the input beam is a first angle {acute over (xcex1)}1 then the angular extent of the scan of the output beam is {acute over (xcex1)}2, whereas {acute over (xcex1)}2={acute over (xcex1)}1*F2/F1. Nevertheless, the throughput increment deteriorates the system resolution as the telescope also reduces the cross section of the scanned beam (in proportion to the ratio between F1 and F2) and eventually increases the spot size.
Another solution for increasing the scanner throughput (without decreasing the resolution) involves using larger electromechanical deflectors or larger acousto-optic deflectors for expanding the beam deflection angle of optical scanning systems, which opposes the general trend toward miniaturizing optical scanning devices. Furthermore, using larger deflectors suffer from low cost/performance ratio and are relatively very complex. For example, increasing the size of mechanical deflectors leads to an increase in their weight, reduced resonance frequescy and limit the scan speed. In creasing the size of an acousto-optic deflector requires more complex transducer configuration and drivers.
The invention described herein provides a solution for the above noted problems associated with increasing wafer inspection speeds.
The present invention enables to increase the throughput of a scanner without decreasing its resolution. The scanning angle expander enables the use of beam deflectors with small physical sizes.
In accordance with another feature of the present invention, the invention involves a scanning angle expander for expanding a scanning angle of an optical scanner, the system includes (i) a beam to multiple beamlets converter and angle conversion optics (also referred to as converting optics), and (ii) multiple beamlet expanding optics (also referred to as expanding optics). It is noted that a focusing optics is usually located between the scanning angle expander and the inspected object (such as a wafer).
The converting optics includes a microlens telescope that includes two microlens arrays (though other types of lenses may be utilized), whereas each microlens of the first microlens array corresponds to a microlens of a second microlens array. The ratio between the focal length of each pair of corresponding microlenses reflects the change between the incidence angle of an input beam that enters a first microlens and the incidence angle of an output beamlet that exits the corresponding microlens of the second microlens array.
The expanding optics converts the plurality of output beamlets to a single output beam having a diameter that is substantially equal to the diameter of the input beam, by performing manipulations in the frequency domain. Conveniently, the multiple output beamlets are provided to a first converging lens (a Fourier transform lens) that performs a Fourier transform of the multiple output beamlets, from the spatial domain to the frequency domain. In other words, the intensity of light at a Fourier plane located at a focal length of the Fourier transform lens reflects the frequency components of the light intensities of the multiple output beamlets that enter the Fourier transform lens.
The light intensity distribution of the multiple output beamlets has various frequency components, reflecting the distance between adjacent output beamlets, and the width of each beamlet. The frequency components are grouped in groups. One of the frequency components groups is a very low frequency component group, that preferably includes a Direct Current (zero frequency) component. The DC component is responsive to the average light intensity of the light intensity distribution. At the Fourier plane these frequency components are reflected by intensity peaks, starting from the lowest frequency component group that includes the DC component. Higher frequency component groups are spatially separated from the lowest frequency component group.
At the Fourier plane, frequency filtering schemes (such as low pass) may be implemented by using spatial filters. The expanding optics blocks (conveniently, by utilizing a spatial filter positioned at the Fourier plane) the high frequency component groups and passes the lowest frequency component group of the light signals to a second lens (inverse Fourier transform lens) that performs an inverse Fourier transform (from the frequency domain to the spatial domain) of the lowest frequency component group of the light signal. In other words, the intensity of light at an Inverse Fourier plane located at a focal length of the inverse Fourier transform lens is a single beam, reflecting an inverse Fourier transform of the lowest frequency component group. Thus, the expanding optics output a single output beam, having substantially the same diameter as the input beam, but oriented at a greater scan angle.